1. Field of the Invention
The present invention generally relates to a wideband sonar beamforming and, more particularly, is concerned with a wideband-to-narrowband conversion method and apparatus for use in wideband sonar beamforming.
2. Description of the Prior Art
Wideband linear frequency modulated (LFM) sonar signals (bandwidth greater than 10 to 15 percent of the carrier frequency) have application in active sonar. In addition to providing increased target strength, they provide application for more robust target classification methodologies. However, there are many advanced high resolution beamforming techniques which are based on phase-shift (not time-delay) beamforming and thus require a spatially narrowband signal across the array. Hence, the use of wideband LFM signals in active sonar is still an active area of research.
Although conceptually straightforward, time-delay beamforming of wideband LFM signals can be problematic, especially when time delays smaller than the time between samples are needed. Present approaches to wideband beamforming, such as exemplified by U.S. Pat. No. 4,544,927 to Kurth et al and U.S. Pat. No. 5,436,872 to Gilmour et al, use a combination of time-delay and phase-shift stages.
Recently, there has emerged an approach to wideband beamforming which converts the wideband signal to a narrowband signal via a spatial resampling filter. This approach is described in Krolik J., xe2x80x9cFocused Wide-Band Array Processing for Spatial Spectral Estimation,xe2x80x9d Advances in Spectrum Analysis and Array Processing, vol. II, ed. Haykin S., Prentice Hall, Inc., NJ, pp. 221-261, 1991. However, the implementation of this methodology would be complex and may be difficult to realize in real time.
Consequently, there is a need for a methodology that is easy to implement in real time and allows phase-shift based beamforming to be used.
The present invention provides a wideband-to-narrowband conversion method and apparatus which is designed to satisfy the aforementioned need. The method and apparatus of the present invention is easy to implement in real time and converts a wideband LFM signal into a spatially narrowband sinusoid, thus allowing any phase-shift based beamforming processing to be used after the conversion process. The present invention applies to widebeam LFM signals and uses multipliers and a weighted bandpass fast Fourier transform (FFT) filter to generate, at a given snapshot or sample in time, the high frequency portion of a Wigner bilinear kernel, evaluated at the center of the array.
Accordingly, the present invention is directed to a wideband-to-narrowband conversion method which comprises the steps of: (a) receiving an acoustic wideband signal and, in response thereto, generating a spatial Wigner bilinear kernel output signal; and (b) receiving and spatially bandpass filtering the spatial Wigner bilinear kernel output signal to remove a wideband low frequency portion as well as any unwanted noise therefrom and retain and output only a narrowband portion thereof.
The present invention is also directed to a wideband-to-narrowband conversion apparatus which comprises: (a) a Wigner bilinear kernel generator means for receiving an acoustic wideband signal and, in response thereto, for generating a spatial Wigner bilinear kernel output signal; and (b) means for receiving and spatially bandpass filtering the spatial Wigner bilinear kernel output signal to remove a wideband low frequency portion as well as any unwanted noise therefrom and retain and output only a narrowband portion thereof.
These and other features and advantages of the present invention will become apparent to those skilled in the art upon a reading of the following detailed description when taken in conjunction with the drawings wherein there is shown and described an illustrative embodiment of the invention.